Anthropogenic CO 2 is a major driver of current environmental change in most ecosystems 1 , and the related ocean acidification (OA) is threatening marine biota 2 . With increasing pCO 2 , calcification rates of several species decrease 3 , although cases of up-regulation are observed 4 . Here, we show that biological control over mineralization relates to species abundance along a natural pH gradient. As pCO 2 increased, the mineralogy of a scleractinian coral (Balanophyllia europaea) and a mollusc (Vermetus triqueter) did not change. In contrast, two calcifying algae (Padina pavonica and Acetabularia acetabulum) reduced and changed mineralization with increasing pCO 2 , from aragonite to the less soluble calcium sulphates and whewellite, respectively. As pCO 2 increased, the coral and mollusc abundance was severely reduced, with both species disappearing at pH < 7.8. Conversely, the two calcifying and a non-calcifying algae * Corresponding authors: Stefano Goffredo -s.goffredo@unibo.it, Tel. +39 051 2094244, Fax +39 051 2094286.; Giuseppe Falinigiuseppe.falini@unibo.it, Tel. +39 051 2099484 . Author Contributions S. G., Z. D., and G. F. conceived and designed research. S. G., F. P., E. C., and B. C. collected the samples and performed the diving fieldwork. L. P., S. F., M. R., and G. F. performed the lab experiments. S. G., F. P., E. C., B. C., L. P., P. F., M. R., and G. F. analysed the data. All authors wrote the manuscript and participated in the scientific discussion. Competing Financial InterestsThe authors declare no competing financial interests. Europe PMC Funders GroupAuthor Manuscript Nat Clim Chang. Author manuscript; available in PMC 2015 January 01. Published in final edited form as:Nat Clim Chang. 2014 July 1; 4(7): 593-597. doi:10.1038/nclimate2241. Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts (Lobophora variegata) showed less severe or no reductions with increasing pCO 2 , and were all found at the lowest pH site. The mineralization response to decreasing pH suggests a link with the degree of control over the biomineralization process by the organism, as only species with lower control managed to thrive in the lowest pH.Several studies on the influence of pH on crystallography and texture of calcified regions are ex situ, short-term experiments on isolated organisms 5 , providing important information, but unrepresentative of natural ecosystems and failing to assess long-term effects 6 . There is a great need of long-term analyses on OA effects on marine ecosystems acclimated to high pCO 2 , as found around CO 2 vents. Vents are not perfect predictors of future oceans, owing to pH variability, proximity of unaffected populations, and co-varying environmental parameters 7 . However, vents acidify seawater on sufficiently large temporal and spatial scales to integrate ecosystem processes 6 , acting as "natural laboratories". In Papua New Guinea vents, reductions in coral diversity, recruitment, abundance, and shifts in competitive interactions are found 8 . I...

Scleractinian coral skeletons are made mainly of calcium carbonate in the form of aragonite. The mineral deposition occurs in a biological confined environment, but it is still a theme of discussion to what extent the calcification occurs under biological or environmental control. Hence, the shape, size and organization of skeletal crystals from the cellular level through the colony architecture, were attributed to factors as diverse as mineral supersaturation levels and organic mediation of crystal growth. The skeleton contains an intra-skeletal organic matrix (OM) of which only the water soluble component was chemically and physically characterized. In this work that OM from the skeleton of the Balanophyllia europaea, a solitary scleractinian coral endemic to the Mediterranean Sea, is studied in vitro with the aim of understanding its role in the mineralization of calcium carbonate. Mineralization of calcium carbonate was conducted by overgrowth experiments on coral skeleton and in calcium chloride solutions containing different ratios of water soluble and/or insoluble OM and of magnesium ions. The precipitates were characterized by diffractometric, spectroscopic and microscopic techniques. The results showed that both soluble and insoluble OM components influence calcium carbonate precipitation and that the effect is enhanced by their co-presence. The role of magnesium ions is also affected by the presence of the OM components. Thus, in vitro, OM influences calcium carbonate crystal morphology, aggregation and polymorphism as a function of its composition and of the content of magnesium ions in the precipitation media. This research, although does not resolve the controversy between environmental or biological control on the deposition of calcium carbonate in corals, sheds a light on the role of OM, which appears mediated by the presence of magnesium ions.

No abstract

The precipitation of calcium carbonate was carried out in the presence of the intraskeletal organic matrix (OM) extracted from Mediterranean corals. They were diverse in growth form and trophic strategy, Balanophyllia europaea and Leptopsammia pruvotisolitary corals, only the first zooxanthellate coraland Cladocora caespitosa and Astroides calyculariscolonial corals, only the first zooxanthellate coral. The results showed that, although the OM marked differences among species, the diverse influence over the calcium carbonate precipitation was evident only for B. europaea. This OM was the most prone to favor the precipitation of aragonite in the absence of magnesium ions, according to overgrowth and solution precipitation experiments. In artificial seawater, where magnesium ions were present, this OM, as well the one from A. calycularis, precipitated mainly a form of amorphous calcium carbonate different from that obtained with SOM from L. pruvoti or C. caespitosa. The amorphous calcium carbonate from B. europaea was the most stable upon heating up to 100 °C and was the one that mainly converted into aragonite instead of magnesium calcite after heating at 300 °C. All this indicated a higher control of B. europaea OM over the calcium carbonate polymorphism than the other species. The influence of SOMs over precipitate morphology turned out to be also species related. In conclusion, this comparative study has shown that the influence of OM on in vitro precipitation of calcium carbonate was not related to the coral ecology, solitary vs colonial and zooxanthellate vs nonzooxanthellate, and suggested that the coral control over biomineralization process was species specific and encoded in coral genes.

Phenotype can express different morphologies in response to biotic or abiotic environmental influences. Mollusks are particularly sensitive to different environmental parameters, showing macroscale shell morphology variations in response to environmental parameters. Few studies concern shell variations at the different scale levels along environmental gradients. Here, we investigate shell features at the macro, micro and nanoscale, in populations of the commercially important clam Chamelea gallina along a latitudinal gradient (~400 km) of temperature and solar radiation in the Adriatic Sea (Italian cost). Six populations of clams with shells of the same length were analyzed. Shells from the warmest and the most irradiated population were thinner, with more oval shape, more porous and lighter, showing lower load fracture. However, no variation was observed in shell CaCO3 polymorphism (100% aragonite) or in compositional and textural shell parameters, indicating no effect of the environmental parameters on the basic processes of biomineralization. Because of the importance of this species as commercial resource in the Adriatic Sea, the experimentally quantified and significant variations of mass and fracture load in C. gallina shells along the latitudinal gradient may have economic implications for fisheries producing different economical yield for fishermen and consumers along the Adriatic coastline.